The field of the invention is that of satellite communications, and more particularly that of controlling the coverage of multiple geographical areas (also known as “spots”) by communication satellites.
In the field of communications, in particular satellite communications, it is desirable for reception quality to be as good as possible. To this end, it is not only necessary that the reception area be covered, but also that the power of the received signals be sufficient.
Of the many types of multizone satellite coverage, there may in particular be cited that which the person skilled in the art knows as multibeam “beam hopping”. Broadly speaking, this type of coverage consists in providing continuous multizone (send and/or receive) coverage with passive antennas, the zones being grouped into cells within each of which only one, so-called “active”, zone is covered at any time and the various zones of the cells being active one after the other, periodically. In particular, this type of coverage enables the whole of the available frequency band to be allocated to an “active” portion of the set of zones during a given period.
Various arrangements provide this type of coverage. They are all based on the same technology whereby each coverage zone is associated with a sending source.
A first arrangement uses first, second, third and fourth send/receive (two-band) antennas containing sources respectively defining first, second, third and fourth zones, each cell then consisting of a first zone, a second zone, a third zone and a fourth zone. In this type of arrangement the mesh available at the level of the sources is sufficiently large to allow the use of wide-aperture (typically 4 to 6λ) sources that are therefore highly directional. This yields very high illumination efficiencies, typically from 75% to 80%. However, the antennas being two-band antennas, the edge of coverage gain (GEOC) cannot be simultaneously optimized for sending and receiving. Moreover, beam hopping being effected by antenna switching, the losses generated at the level of the connecting guide between each source and the switch are high.
A second arrangement reproduces the preceding arrangement with the number of antennas doubled to four send antennas and four receive antennas. In this type of arrangement the mesh being substantially identical to that of the preceding arrangement it is therefore possible to obtain very high illumination efficiencies, typically from 75% to 80%. The antennas in this case being optimized in each frequency band, it is therefore possible to optimize the edge of coverage again (GEOC) simultaneously for sending and for receiving. However, using eight antennas introduces significant layout constraints. Moreover, beam hopping also being effected by antenna switching, the losses generated at the level of the connecting guides between each source and the switch are high.
A third arrangement is based on the first arrangement and reduces the number of antennas to three. Here the available mesh is slightly smaller than in the above two arrangements, so that the sources have an aperture of the order to 3 to 5λ and are therefore slightly less directional. The illumination efficiency remains very acceptable and layout constraints are greatly reduced. However, beam hopping is still effected by antenna switching, so the losses generated at the level of the connecting guides between each source and the switch are high. Moreover, the mesh being tighter, the carrier/interference (C/I) ratio is degraded (the interfering signals “I” are generated by the other sources that are operating in the same frequency band and with the same polarization as the active zone).
A fourth arrangement consists in using only one send antenna and one receive antenna. Beam hopping now being effected by switching within the same antenna, the losses generated at the level of the connecting guides between each source and the switch are low. However, defining all the zones with a single antenna imposes a very tight mesh, so that the sources have an aperture of the order of 1.2 to 1.5λ and are therefore only very slightly directional. The illumination efficiency is then very low (typically from 35% to 40%), which imposes oversizing of the antenna reflectors and the antennas, which can lead to technology problems, especially if the satellite is operating in the “Ka” frequency band. The edge of coverage gain (GEOC) is therefore reduced by 3 to 4 dB compared to the preceding arrangements, and the “roll-off” (the gain variation over the whole of the multizone coverage, and to be more precise the difference between the maximum gain in each zone and the EOC gain) is very high, typically of the order of 8 to 12 dB compared to the 4 to 6 dB of the preceding arrangements.
Thus no prior art arrangement gives entire satisfaction in terms of multizone coverage by “beam hopping”.
The situation is substantially the same in respect of the other types of multizone coverage and in particular multizone coverage by static deviation of multiple beams and multizone coverage by dynamic deviation of one beam.